1. Field of the Invention
The field of the invention is pipe storage devices and in particular, pipe storage devices that rotate pipe or cable about a vertical axis.
2. Description of the Related Art
In the oil and gas industry, flexible and semi-flexible tubular goods and cables are stored and transported on spools or carousels. The rotating axes of spools are oriented either horizontally or vertically while the rotating axes of carousels are oriented vertically.
Carousels have been used for many different purposes over the years. For example, U.S. Pat. No. 1,819,656 shows the use of a carousel to revolve a stage. U.S. Pat. No. 3,968,961 shows a merry-go-round carousel. U.S. Pat. No. 2,964,144 and US Patent Publication 2003/0145760 show carousels used for rotatable aircraft storage hangars. U.S. Pat. No. 6,745,699 shows a carousel used to rotate production parts on a production line. The subject of this invention relates to carousels used to store and unspool pipe or cable, as would be typically in the oil and gas industry.
Most carousels rotate on uneven surfaces because it is not generally possible or practical to provide a truly flat surface. The flatness can vary from approximately +/−0.10 inch to +/−3 inches or more and the possible variation in surface flatness increases as the size of the carousel increases. The carousel of this invention may be either in a fixed location, such as in a manufacturing building or it may be on the deck or inside of a marine vessel. If a carousel is built and installed in a building, it would typically rotate on a concrete based, or other hard surface, which generally could be level to within +/−0.25 inch, but possibly as much variation as +/−0.5 inch, or more. For applications involving mounting a carousel on a marine vessel, the variation in support surface flatness can be significant. Moreover, a deck of a vessel, from which pipes or cables are deployed offshore, can be curved or can flex with the variable forces that are encountered in the offshore environment. Marine vessels typically have flexibility designed in, to prevent stress damage from the changing and considerable ocean forces.
U.S. Pat. Nos. 3,804,111 and 3,941,146 show the use of a carousel for purposes of storing drill pipe. Both U.S. Pat. Nos. 3,804,111 and 3,941,146 also show the vertical displacement of wedge shaped sectors to raise the entire carousel sector in response to an uneven support surface. U.S. Pat. No. 3,965,713 shows the use of a carousel to store and unspool pipe during offshore pipe lay operations in preparation for oil or gas production.
There can be substantial lengths and weights of tubular goods that are coiled onto the drill pipe and offshore pipe carousels. As the lengths and loads increase, the support structure of the carousel can become extremely important. As the lengths of the pipe to be stored increases, the overall diameter of the carousel must also increase. As the diameter of the carousel increases, it becomes probable that the underlying support surface, which supports the carousel mechanism and the weight of the pipe on the carousel, will be uneven and will not be perfectly flat. Carousels are typically supported by casters that rotate about circular rails. If the underlying support is uneven, the casters can rise up and lose contact with the supporting circular rails. If that happens, the load is transferred to an adjacent caster or a caster that is in contact with the rails. Because of the extreme loads that are carried by the carousel, the concentrated loads placed on those casters that are in contact with the rail can exceed the capacity of the casters, which can cause the load bearing casters to fail. The problem can then become aggravated because if one or more casters fail, the pipe weight can then immediately and catastrophically be transferred to other casters that are in contact with the rails. The failure cascades by then causing the remaining casters to be exposed to the load that is no longer supported by the failed casters.
It may also be desirable to move flexible pipe from the manufacturing location to a pipe lay vessel, from which the flexible pipe may then be installed into the subsea water column. The decks of barges and work vessels are somewhat flexible by design, to respond to wave and hydrodynamic forces that are imposed on them when they are operating in the oceans, seas and the challenging offshore environment. The flexing causes the underlying support to become uneven, which can again cause the problem of all casters not being in contact with the circular rails.
Additional problems can occur when extremely large diameter carousels are constructed. Because of the extreme transverse loads that occur when the carousel is rotated during the spooling or unspooling of pipe, it is not practical to use a conventional center “kingpin”. The transverse loads that are placed on the center kingpin would require that the kingpin be extremely large. Furthermore, the radius of the center hub must be no less than the minimum bend radius of the flexible pipe that is wound about the center of the carousel.
The large diameter necessary to accommodate long lengths of relatively large diameter pipe also creates challenges related to the mechanism that is used to rotate the carousel. Motors may be attached to individual casters, as shown in U.S. Pat. Nos. 3,804,111 and 3,941,146. As the number of casters increases, and the size and weight of the carousel increases, the number of individual motors can become unwieldy and unreasonably expensive. Gears may be attached about the circumference of the carousel, with a motor engaged with the track, as shown in U.S. Pat. No. 3,965,713. This requires a very large, heavy and expensive gear to be used. A motor, attached to a rotating gear, may also be used to engage a chain that encircles the entire carousel, as shown in U.S. Pat. No. 6,745,699. Unfortunately, when a chain encircles the entire carousel, it becomes extremely heavy and expensive, and if the weight of the chain sags it can cause serious operational problems.